Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock_gettime()

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Mon, Aug 15, 2016 at 09:33:49AM +0100, Mel Gorman wrote:
> On Mon, Aug 15, 2016 at 09:49:05AM +0200, Giovanni Gherdovich wrote:
> > > mmtest benchmark results are below (full compare-kernels.sh output is in attachment):
> > > 
> > > vanila-4.7            revert                prefetch              patch
> > > 4.74 (  0.00%)        3.04 ( 35.93%)        4.09 ( 13.81%)        1.30 ( 72.59%)
> > > 5.49 (  0.00%)        5.00 (  8.97%)        5.34 (  2.72%)        1.03 ( 81.16%)
> > > 6.12 (  0.00%)        4.91 ( 19.73%)        5.97 (  2.40%)        0.90 ( 85.27%)
> > > 6.68 (  0.00%)        4.90 ( 26.66%)        6.02 (  9.75%)        0.88 ( 86.89%)
> > > 7.21 (  0.00%)        5.13 ( 28.85%)        6.70 (  7.09%)        0.87 ( 87.91%)
> > > 7.66 (  0.00%)        5.22 ( 31.80%)        7.17 (  6.39%)        0.92 ( 88.01%)
> > > 7.91 (  0.00%)        5.36 ( 32.22%)        7.30 (  7.72%)        0.95 ( 87.97%)
> > > 7.95 (  0.00%)        5.35 ( 32.73%)        7.34 (  7.66%)        1.06 ( 86.66%)
> > > 8.00 (  0.00%)        5.33 ( 33.31%)        7.38 (  7.73%)        1.13 ( 85.82%)
> > > 5.61 (  0.00%)        3.55 ( 36.76%)        4.53 ( 19.23%)        2.29 ( 59.28%)
> > > 5.66 (  0.00%)        4.32 ( 23.79%)        4.75 ( 16.18%)        3.65 ( 35.46%)
> > > 5.98 (  0.00%)        4.97 ( 16.87%)        5.96 (  0.35%)        3.62 ( 39.40%)
> > > 6.58 (  0.00%)        4.94 ( 24.93%)        6.04 (  8.32%)        3.63 ( 44.89%)
> > > 7.19 (  0.00%)        5.18 ( 28.01%)        6.68 (  7.13%)        3.65 ( 49.22%)
> > > 7.67 (  0.00%)        5.27 ( 31.29%)        7.16 (  6.63%)        3.62 ( 52.76%)
> > > 7.88 (  0.00%)        5.36 ( 31.98%)        7.28 (  7.58%)        3.65 ( 53.71%)
> > > 7.99 (  0.00%)        5.39 ( 32.52%)        7.40 (  7.42%)        3.65 ( 54.25%)
> > > 
> > > Patch works because we we update sum_exec_runtime on current thread
> > > what assure we see proper sum_exec_runtime value on different CPUs. I
> > > tested it with reproducers from commits 6e998916dfe32 and d670ec13178d0,
> > > patch did not break them. I'm going to run some other test.
> > > 
> > > Patch is draft version for early review, task_sched_runtime() will be
> > > simplified (since it's called only current thread) and possibly split
> > > into two functions: one that call update_curr() and other that return
> > > sum_exec_runtime (assure it's consistent on 32 bit arches).
> > > 
> > > Stanislaw
> > 
> 
> Is this really equivalent though? It updates one task instead of all
> tasks in the group and there is no guarantee that tsk == current.

Oh, my intention was to update runtime on current.

> Glancing at it, it should monotonically increase but it looks like it
> would calculate stale data.

Yes, until the next tick on a CPU, the patch does not count partial
runtime of thread running on that CPU. However that was the behaviour
before commit d670ec13178d0 - that how old thread_group_sched_runtime()
function worked:

 /*
- * Return sum_exec_runtime for the thread group.
- * In case the task is currently running, return the sum plus current's
- * pending runtime that have not been accounted yet.
- *
- * Note that the thread group might have other running tasks as well,
- * so the return value not includes other pending runtime that other
- * running tasks might have.
- */
-unsigned long long thread_group_sched_runtime(struct task_struct *p)

Stanislaw